The dimensions of rail road cars are constrained by a number of geometric considerations. First, on tangent track (that is, straight track) a rail road car can not be too wide, otherwise it may foul the sides of bridges, tunnels, roadside fittings such as switches or signals, or other cars of the same size passing on an adjacent track. Similarly, rail cars cannot be taller than the minimum regulated heights of the lowest bridges or tunnels on the tracks along which it is to travel. Third, the weight a car can carry is limited by the capacity of the tracks, rails and road bed over which it is to travel.
With reference to FIGS. 1a, 1b, 1d and 1e, on curved track, the relationship between length and width is important. Traditionally, single unit rail road cars A20 have had a car body supported by a rail car truck A22, A24 at either end. The mounting to a standard two axle, four wheel truck is at a pivot at the truck center, A26. The cars are connected at a releasable coupler A28 in the commonly known manner. When such a car passes through a curve trucks A22, A24 follow the arc indicated by the track centerline, S1, while the car body centerline between the truck centers forms a chord κ of the arc. Chord κ subtends an angle α1 of arc S1. This is shown, with exaggerated proportions, in FIG. 1a. The track center line radius is indicated as R1. At midspan between the trucks, the inside edge of the car follows a circular arc having a radius of curvature indicated as the limiting inside minimum radius R2. Car A20 is shown as having overhanging end portions A30 and A32 that extend longitudinally outboard of the respective truck centers. As car A20 passes through a curve the extreme outside corners of end portions A30 and A32 will follow along an outer radius, namely the limiting minimum outside radius indicated as R3.
For any curve, the longitudinal center line of the car, CL, at mid-span between the trucks will lie some distance, δ, inward from the center of the track, as indicated by δ1. This distance δ depends on the radius of curvature, R1 of the tracks, and the distance between truck centers, L1. As shown in FIG. 1a, for a given dimension L1, δ increases as the radius of curvature decreases, as indicated by R4. Alternatively, for a fixed track radius R1, as the truck center distance L1 increases, δ also increases. The left hand example of FIG. 1a demonstrates this. For a track having a radius of curvature R4, the arc is identified as S2. Placing two of rail road cars A20 on this track, the chord length remains κ but the subtended angle, α2, is larger than α1, and the distances between the inner and outer clearance radii, R5 and R6, is greater than between R2 and R3, with a consequent increase in δ form δ1 to δ2.
In North American service, the relationship of rail road car width and length, and the corresponding necessary reductions in width required as truck center distance increases are set out by the American Association of Railroads (AAR) in various AAR standards. Cars to be used in interchangeable service are required to conform to the AAR standards. For all cars, including AAR plate ‘C’ cars, the limiting centerline track radius, R1, is a standard minimum dimension of 5300.375 inches. For plate ‘C’ cars, the limiting minimum inside radius, R2, is determined on the basis of a car (“the base car”) having a truck center spacing of 46′-3″ (555 inches), and a maximum car width of 10′-8″ (128 inches). For this standard car, δ1 is roughly 7.25 inches, so R2 is roughly 5229.12 inches For plate ‘C’ cars the limiting minimum outside radius, R3, is defined as being greater than R1 by the same amount as R2 is less than R1. Thus, adding the 7.25 inch offset, plus half of the car width, namely 64 inches, gives an R3 of 5371.63 inches.
If car A20 is not to foul adjacent cars or adjacent structures while passing through curves, as the truck center length increases beyond 46′-3″, the width of the car must decrease correspondingly so the inside of the car at mid-span between the trucks of the car does not cut to the inside of R2. The allowable width of a car for a given truck center distance can be calculated from this datum case. A different standard applies for auto-rack rail road cars, but the principles are the same. In AAR specification M-950-A-99, the maximum width of a bi-level auto-rack car having a length of 90′ over the strikers is given as 119″ at mid span, and 121″ at the strikers. Typically such an auto-rack has truck centers on either 64′ or 66′ spacing. The limiting minimum inside radius, R2, for this car is 5226.06 inches and the limiting minimum outside radius, R3, is 5373.27″. The outside extreme corners A30, A32 must stay within R3. In some cases, for long overhangs, the ends of the car must be narrowed.
Similarly, some types of inter-modal well cars are used for carrying containers, or for carrying highway trailers or a combination of the two. The well must be wide enough to accommodate either the highway trailers or the containers, as may be required. Center beam cars, such as are commonly used for carrying stacked bundles of lumber must have wide enough bunks to carry standard widths of bundles.
Auto-rack rail road cars must be wide enough not only to carry automobiles, but they also must be wide enough to allow space for persons loading and unloading the automobiles to open the automobile doors and get in and out of the automobiles. The person loading the automobiles must also have sufficient space to walk beside the automobiles. When the clearance allowed is too small, the loading personnel may inadvertently damage the finish of the automobiles, giving rise to damage claims. Alternatively, it may be that it is helpful, or necessary, to allow a clearance envelope to accommodate motion of the lading during travel. In each case, it is helpful to lengthen the car to increase lading, but such lengthening is limited by the need to maintain a car body width.
Conventionally, articulated rail road cars have two or more rail car units permanently connected to each other such that one rail car truck is shared between two adjoining rail car units. Typically, an articulated rail road car having a number of rail car units ‘n’ is supported on ‘n+1’ trucks. An articulation connection is a permanent connection unlike a hitch or standard releasable coupling that can be coupled and uncoupled each time a new train consist is made up in a shunting yard. By contrast, an articulated connector, once assembled, tends only to be taken apart during repair or replacement at a workshop, and is considered a permanent connection.
In FIG. 1b, an articulated rail road car B20 has first and second rail car units B22 and B24. They are joined at their respective inboard ends B26 and B28 by an articulation connection B30 mounted directly above the truck center of a four wheel truck B32 that is shared between units B22 and B24. The track radius is shown as R1. The allowable inside radius is shown as R2. The allowable outside radius is shown as R3. The extreme corners of outboard ends B34 and B36 fall just within radius R3. When articulated truck B32 is used, while the inside of the body of car B20 is tangent to radius R2, there is clearance between the outermost extremities of inboard ends B26 and B28. This occurs because truck B32, is constrained to follow the tracks, and there is no overhang of either unit B22 or unit B24 at truck B32 comparable to the overhang at each of the outboard ends B34 and B36.
Further, in the example of FIG. 1b, a vertically downward shear load is passed from each of car units B22 and B24 into articulation connection B30, and then directly into the truck bolster of truck B32. That is, each of the car units B22 and B24 approximates a span having a simple support at each end into which the vertical shear load, but no bending moment, is passed for reaction through the trucks, and ultimately, by the road bed lying underneath the rails. It will be appreciated that in a multi-unit articulated car having three or more car units, at least one unit will have an articulation connection under both ends.
FIG. 1d shows a three-unit articulated rail road car C20, having a middle rail car unit C22 and end rail car units C24 and C26. As in FIG. 1b, rail road car C20 is shown on a section of track having centerline radius R1, minimum inside clearance radius R2, and minimum outside clearance radius R3. As before, the truck center distance is L1, and the mid-span lateral inset of the longitudinal centerline of rail car unit C22 (and, in this example, also of rail car units C24 and C26), is again 61. As above, car unit C22 is joined to car units C24 and C26 by respective articulated connectors C28 and C30 whose points of articulation lie directly above corresponding shared trucks C32 and C34. It can be seen that the outside corners C36 and C38 of car unit C22, and corners C40 and C42 of car units C24 and C26 lie well inward of outside radius R3.
The rail road cars shown in FIGS. 1a, 1b and 1d have pivoting, two axle, four-wheel trucks that pivot relative the longitudinal centerlines of the respective car bodies. This permits the truck to run along the arc while the car body forms a chord of the arc, the chord meeting the track centerline at an angle. Single truck railcars are known, particularly in light-weight service as for passenger car train sets where the individual axle loading levels tend to be low relative to the customary load limits of freight cars. The use of single axle trucks in an articulated freight car may tend to be disadvantageous.
First, a single axle truck is generally fixed relative to the car body. If allowed to pivot freely in the manner of a double axle truck, a single axle truck would not necessarily continue to follow the rails. However, as car length increases, fixed orientation single axle trucks face an increasing angle of attack relative to the rails when running through a curve. Consequently, single axle trucks tend not to be recommended for rail cars having a separation of more than about 39 feet between trucks. However, the issue of having to reduce the width of the rail road car occurs when the truck centers are already more than 46 ft. 3 in. apart. Second, a single axle truck cannot, in general, carry the same load as a double axle truck having comparable wheels. While single axle trucks may be suitable for the carriage of short, light passenger cars, the length and greater lading of freight cars tends to require double axle trucks.
As noted, in the arrangement shown in FIG. 1b, the articulated rail car units are able to pivot relative to the shared truck, and relative to each other. There is a permanent articulated connector, having a male member and female socket. The articulated connector has a pivot axis that is generally located directly above the center of the shared truck, such that the pivot point of the socket is coincident with the truck center when viewed from above. In this type of arrangement, the pivot point tends always to lie directly above the centerline of the track. One type of articulated connector is shown in U.S. Pat. No. 4,336,758 of Radwill, issued Jun. 29, 1982, in which the main pin is nominally vertical. Another type of articulation connection is shown in U.S. Pat. No. 5,271,511 of Daugherty, Jr., issued Dec. 21, 1993 in which a main pin, in the nature of a removable shaft, is nominally horizontal.
One advantage of articulated connections is that they tend to take up less longitudinal space than common interchangeable couplers. In one application, a number of large automobile manufacturing facilities have a loading siding length that is chosen to handle a string of cars, whether articulated or otherwise, or some combination thereof, up to a limit of 500 ft. in length. One automobile manufacturer would like to be able to load 4 automobiles of a type having a length of 239″ (or less), or five compact automobiles on a single auto rack car, or, in the case of an articulated car, on a single car unit. When standard releasable couplers are used on stand alone cars, a 500 ft siding can accommodate 5 rail cars with an overall length of roughly 470′, with a total capacity on a single deck level of 20 automobiles of 239 inch length each. A pair of three-pack articulated rail road cars made according to the present invention may tend to permit a six unit rail road car to be accommodated on a 500 ft siding with a total capacity on a single deck level of 24 automobiles of 239 inch length each.
Another advantage is that articulated couplers tend to be slackless couplers. This tends to reduce the longitudinal shock load transmitted during run-in and run-out, and during shunting. Other types of slackless coupling exist other than articulated couplings. For example, it is possible to use a draw bar between cars, as shown, for example, in U.S. Pat. No. 4,929,132 of Yeates et al., issued May 29, 1990.
A draw bar is a bar of fixed length that is connected at pivot points at either end to adjacent rail car units on either side. A draw bar reduces the clearance required between the car units as compared to releasable couplers, but cannot be used to transmit a shear load. That is, it may not tend to be advantageous to try to pass a vertical shear load through a draw bar. Thus use of a draw bar rather than an articulated connector generally requires that there be an adjacent truck mounted to each of the rail car units, with the consequent increase in weight, length, maintenance, and expense.